Twist drill for deep hole drilling

ABSTRACT

A twist drill for deep hole drilling comprises a cutting portion, margins (2) and helical flutes (3) designed for chip removal. The front-end wall (4) of the helical flute (3) comprises two conjugate sections (7, 8). The first section (7) intersecting with the margin (2) forms with the generatrix of the twist drill outer cylindrical surface an angle (ψ) which is smaller than 90°. The angle (δ) of inclination of the tangent at any point of the second sections (8) of the front-end wall (4), to the generatrix (9) of the twist drill outer cylindrical surface is selected proceeding from the relationship: 
     
         125°&gt;δ&gt;(57°-ω)·K+90°, 
    
     where: ω is the angle of helix of the helical flute (3) relative to the twist drill axis at the point under consideration. 
     
         K=1.4-2.1.

FIELD OF THE INVENTION

The present invention relates to machining of metals and othermaterials, and more specifically to a twist drill for deep-holedrilling.

Drilling is one of the most widely, known operations in metal working.Usually this operation is characterized by low productivity andsubstantial consumption of tools. Drills comprise approximately 30% ofthe produced end-cutting tools. An especially low productivity ispeculiar to drilling holes more than ten diameters deep, i.e. to thedeep hole drilling. Therefore, there exists a problem of increasing theproductivity of drilling such holes.

BACKGROUND OF THE INVENTION

State and Analysis of the Prior Art Known in the prior art is a twistdrill for deep hole drilling, comprising a cutting portion, margins andhelical flutes designed for removal of chips (SU, A, 701,743). Each ofthe helical flutes is arranged at an angle relative to the twist drillaxis front-end wall nearest the cutting portion of the drill andconjugated with a flute bottom which is parallel with the twist drillaxis and which is conjugated with a rear-end wall.

The known twist drill makes it possible to drill holes with a depth ofup to 50 diameters without periodically withdrawing the drill due to anenlarged angle of flute helix (ω=45°-60°) and an inclination of theflute front-end wall relative to the twist drill axis through an angle δwhich is greater than 90°, the front-end wall being made reactilinear.Productivity in drilling deep holes with such a twist drill is higher incomparison with that of the widely known twist drills used for the deephole drilling. The combinations of correctly selected three main designparameters, i.e. rational dimensions, shape and angle of flute helixmade it posible not only to eliminate periodical withdrawals of thedrill but also to substantially improve the torsional stiffness of atwist drill. This is explained by the fact that the provision of areliable removal of chips from the hole being drilled allows thedimensions of the helical flutes to be substantially reduced and thediameter of a drill core to be increased, as there is no need to havethe helical flutes of a large volume for accumulation of chips. Theknown twist drill possesses a comparatively high stiffness combined witha reliable chip removal. This is achieved by reducing the angle of flutehelix ω at the expense of introducing an angle of inclination δ of thefront-end wall. The occurence of a force pressing the element of a chipagainst the wall of a hole made it possible to improve the conditions ofthe chip removal.

However, due to the fact that the front-end wall of a helical flute hasthroughout its full heights one and the same angle of inclinationrelative to the twist drill axis, the conditions of the chip removal onthe entire length of the front-end wall are not equal. So, theinclination in the middle portion of the helical flute in the place ofcontact with large elements and curls of chips is useful, as it improvesthe conditions of the chip removal. The same inclination at theperiphery of the helical flute in the place of contact with pulverulentparticles and fine fragments of chips is unfavourable, as it impairs theworking conditions of the known twist drill and reduces its endurance.This is associated with the fact that in a cross-section of the twistdrill the front-end wall of such a helical flute is formed near theperiphery by a convex curve, i.e. by the convolute and at the point ofintersection with the margin said front-end wall forms an obtuse angle.This leads to jamming of the fine and pulverulent particles of chips inthe clearance between the drill and the hole, and to their sticking tothe margins, as well as impairs the twist drill endurance and increasesthe torque.

SUMMARY OF THE INVENTION

The present invention is essentially aimed at providing a twist drillfor deep hole drilling having such an embodiment of helical flutes whichwill provide a reliable chip removal in combination with the maximumpossible torsional stiffness of the twist drill, and will eliminate thesticking of a drilled out material to the margins.

This aim is attained by a twist drill for deep hole drilling, comprisinga cutting portion, margins and helical flutes designed for chip removal,each of which is arranged angularly relative to the twist drill axisfront-end wall nearest the cutting portion of the drill and conjugatedwith a flute bottom which is parallel with the twist drill axis andwhich is conjugated with a rear-end wall, according to the invention,the front-end wall comprises two conjugate sections a first of whichintersects the margin and forms the generatrix of the twist drill outercylindrical surface at an angle which is less than 90°, while an angleof inclination of the tangent at any point of the second sections of thefront-end wall to the generatrix of the twist drill outer cylindricalsurface is selected from the relationship:

    125°>δ>(57°-ω)·K+90°,

where: ω is the angle of flute helix relative to the twist drill axis atthe point under consideration,

    K=1.4-2.1.

The length of a projection of the first section on the planeperpendicular to the axis of the twist drill may suitably comprise0.05-0.3 of helical flute depth.

This will prevent jamming of the fine particles of chips between thetwist drill and the hole wall without impairing the chip removingproperties of the helical flute.

The lead of a helical flute is preferably selected according to thefollowing formula:

    P=(2-2.6) d,

where: d is the diameter of a twist drill.

This provides a reliable chip removal along with the preservation of acomparatively high stiffness of the twist drill.

The twist drill for deep hole drilling made according to the presentinvention insures an uninterrupted removal of chips from the hole beingdrilled, thereby making it possible to drill deep holes withoutperiodically withdrawing the proposed drill, i.e. in one pass. Theproposed drill possesses an increased torsional stiffness which makes itpossible to employ increased cutting speeds and feeds. The proposedtwist drill may be used on any drilling machines without theirmodernization. Besides, the manufacturing methods used in production ofthe twist drill embodiment according to the invention practically do notdiffer from the manufacturing methods used in producing of widely knowntwist drills.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the invention will be described in greater detail with reference toa specific embodiment thereof, taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 illustrates a general view of a twist drill for deep holedrilling, according to the invention;

FIG. 2 is a section taken along the line II--II of FIG. 1 illustrating adiagram of forces acting on an element of the chip.

DETAILED DESCRIPTION OF THE INVENTION

A twist drill for deep hole drilling comprises a cutting portion 1 (FIG.1), margins 2 and helical flutes 3 designed for the chip removal.

Each helical flute 3 is formed by a front-end wall 4 arranged at anangle δ (FIG. 2) to the twist drill axis and conjugate with a flutebottom 5 parallel with the twist drill axis and conjugate with arear-end wall 6.

The front-end wall 4 comprises two conjugate sections 7,8. The firstsection 7 intersects with the margin 2 and forms with a generatrix 9 ofthe twist drill outer cylindrical surface an angle ψ<90°. The length ofa projection of the first section 7 of the front-end wall 4 on the planeperpendicular to the twist drill axis comprises 0.05-0.3 of a depth h ofthe helical flute 3. Due to such geometric parameters of the firstsection 7, a sharpening is formed at the periphery of the drill in thecross-section and a created normal pressure force N is directed so thatit breaks fine particles 10 of chips away from a hole wall 11 andprevents them from being jammed in the clearance between the twist drilland the hole wall. The sharpening at the point of transition of thefront-end wall 4 of the helical flute 3 into the marging 2 improves thetwist drill endurance by several times when compared with the twistdrills having at this point the angle ψ>90°. If the dimension of thefirst section 7 at which the sharpening is formed, is too small, i.e.commensurable with the dimensions of pulverulent and fine particles 10of chips, the endurance of a twist drill decreases due to the stickingof these fine particles 10 to the margins 2. It is not rational to makethis section 7 less than 0.05 of the depth h of the halical flute 3. Ifthe dimension of the first section 7 is over 0.3 of the depth of thehelical flute 3, the medium and coarse elements 12 of chips come incontact with the front-end wall 4 at the points where a radial componentR of the normal pressure force N is too small or is directed toward thetwist drill axis which reduces the chip removing properties of thehelical flutes 3.

The second section 8 conjugate with the bottom 5 of the helical flute 3is made so that the angle δ of inclination of the tangent, passingthrough a conjugate point of the first and second sections 7, 8 of thefront-end wall 4, to the generatrix 9 of the twist drill outercylindrical surface is selected from the relationship:

    125°>δ>(57°-ω)·K+90°,

where: ω (FIG. 1) is the angle of helix of the helical flute 3 relativeto the twist drill axis at the point under consideration,

    K=1.4-2.1.

The presence of the angle δ (FIG. 2) brings about a radial component ofthe normal pressure force N with which the twist drill acts on theelement 12 of the chip. A vector of the radial component R being addedtogether with the vector of a centrifugal force W presses the element 12of the chip against the hole wall 11 which increases the force offriction of the chips on the wall of a drilled hole and, consequently,improves the conditions of chip removal. When the angle δ is increasedfrom 90° to 120° the conditions of chip removal are improved. When theangle δ>125°, there occur negative phenomena which reside in that theforce N pressing the chips against the hole wall increases to such avalue that when drilling in steel, cast irons and other structuralmaterials annular grooves are caused on the hole walls 11 under theaction of chips because of which the chip removal is disturbed.

Conducted experiments have proved that in order to ensure the reliableremoval of chips when the front-end wall 4 has no inclination, it isdesirable that the angle ω (FIG. 1) of helix of the helical flutesrelative to the twist drill axis be in a range of 57° to 62°. Theinclination of the front-end wall 4 (FIG. 2) makes it possible to ensurea reliable chip removal with the angle ω reduced to 52°-54°. Thisincreases the stiffness of a twist drill as the stiffness is associatedwith the angle ω.

As the chips do not accumulate in the drill helical flutes 3, it becomespossible to reduce the dimensions of these flutes 3 which provides anadditional increase in the twist drill stiffness. A width b of thehelical flute 3 is reduced at the periphery of the twist drill andcomprises (0.2-0.3) P, where P (FIG. 1) is the lead of the helical flute3. The lead P of the helix is selecting proceeding from the conditionsof the uninterrupted removal of chips. When the depth of drilling isover 50 diameters and the angle δ=95°-125° the lead of helix is selectedproceeding from the condition that P=(2-2.6)d, where d is the drilldiameter. When the lead P of helix (at a greater angle ω) is smaller thestiffness if the twist drill is reduced, while at P>2.6d the chipremoval is disturbed.

Industrial Applicability

To the most advantage the present invention may be used for deep holedrilling in parts of motor vehicles, tractors and hydrojets.

We claim:
 1. A twist drill for deep hole drilling, comprising a cuttingportion, margins and helical flutes designed for chip removal, each ofsaid flutes is arranged at an angle relative to the twist drill axisfront-end wall nearest the cutting portion of the drill and isconjugated with a flute bottom which is parallel with the twist drillaxis and which is conjugated with a rear-end wall, said front-end wallcomprising two conjugate sections, the first section intersecting withthe margin to thereby form, with the generatrix of the twist drill outercylindrical surface, an angle of less than 90°, while the angle ofinclination of the tangent at any point of the two conjugate sections ofthe front-end wall, to the generatrix of the twist drill outercylindrical surface is selected from the relationship:

    125°>angle of inclination>(57°-w)K+90°,

wherein w is the helix angle of the helical flute relative to the twistdrill axis at the point under consideration, and

    K is from 1.4 to 2.1.


2. The twist drill of claim 1 wherein the length of a projection of thefirst section on a planar perpendicular to the twist drill axis is0.05-0.3 of the depth of the helical flute.
 3. The first drill of claim1 wherein the lead (P) of the helical flute satisfies the followingformula:

    P=(2-2.6)d,

wherein d is the diameter of the twist drill.